A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to improve imaging of mask patterns, it is well known to apply techniques such as mask-biasing (in which feature sizes are changed to pre-compensate for imaging errors), serifs (which are additions to pattern features to improve imaging of ends and corners), and optical proximity correction (OPC) (in which sub-resolution assist features of various types are added to compensate for diffraction effects due to the presence or absence of neighboring features). In particular, assist features are applied in the proximity of isolated or semi-isolated features to increase the depth of focus (DOF) of those features. However, improvements in DOF are made at the expense of exposure latitude (EL) so that a compromise between these figures of merit has to be made. In some cases, exposure latitude is sufficiently small that assist features cannot be used.